Evolutionary Relationships With Other Phyla

Arthropods were traditionally linked with the phylum Annelida in the phylogenetic clade Articulata because both are metameric phyla, but more recent molecular analyses provide no support for a particularly close relationship. Instead, arthropods seem to be linked with other phyla that must shed their cuticle during ecdysis to grow. This clade of "Ecdysozoa" also includes the phyla Tardigrada, Onychophora, Nematoda, and Nemato-morpha, along with the more distantly related Priapulida and Kinorhyncha. At a greater phylogenetic distance from Ecdysozoa is the other major protostomate clade consisting of the phyla Rotifera, Annelida, Mollusca, Bryozoa (Ectoprocta), Brachiopoda, and Phoronida. Pentastomida is listed by some authors as a separate phylum with links to Arthropoda through the clade Ecdysozoa, but here it is included as a class within the arthropod subphylum Crustacea.

Within Ecdysozoa, the three closest phyla are Arthropoda, Tardigrada, and Onychophora. This conclusion is based in part on molecular studies using 185 rRNA. Fossil evidence from the mid-Cambrian (~520 mya) suggests that onychophoran-like limbs developed in aquatic invertebrates and may have served as a preadaptation for terrestrial life. Similarities in morphology and physiology also seem to link these phyla. For example, tardi-grades possess striated muscles, paired ventral nerve cords, and a large hemocoel. In addition to these characteristics, ony-chophorans have the following arthropod-like features: a tracheal respiratory system, mandible-like mouth appendages, cardiac ostia, an excretory system comparable to the green gland of crustaceans, one pair of antennae, and similar defensive secretions produced by repugnatorial glands. Both Onychophora and Tardigrada, however, have some decidedly non-arthropod-like characteristics (e.g., nonjointed legs). Onychophora, Myri-apoda, and Hexapoda are grouped by some systematists into Uniramia, a single phylum of arthropod-like animals having a single branch (ramus) of body appendages. According to this theory Uniramia is phylogenetically isolated from Crustacea and Chelicerata, but all are in the superphylum Arthropoda.